剑指offer——面试题49：不使用atoi库函数实现把字符串转换成整数

下面是参考代码：

enum Status {kValid = 0, kInvalid};
int g_nStatus = kValid;

int StrToInt(const char* str)
{
g_nStatus = kInvalid;
long long num = 0;

if(str != NULL && *str != '\0')
{
bool minus = false;
if(*str == '+')
str ++;
else if(*str == '-')
{
str ++;
minus = true;
}

if(*str != '\0')
{
num = StrToIntCore(str, minus);
}
}

return (int)num;
}

long long StrToIntCore(const char* digit, bool minus)
{
long long num = 0;

while(*digit != '\0')
{
if(*digit >= '0' && *digit <= '9')
{
int flag = minus ? -1 : 1;
num = num * 10 + flag * (*digit - '0');

if((!minus && num > 0x7FFFFFFF)
|| (minus && num < (signed int)0x80000000))
{
num = 0;
break;
}

digit++;
}
else
{
num = 0;
break;
}
}

if(*digit == '\0')
{
g_nStatus = kValid;
}

return num;
}